The present invention relates to a process for forming a soft, flexible polyurethane film by powder coating and, more particularly, to a process for forming a low modulus essentially non-crystalline polyurethane film which is free of pin holes.
Although powder coating techniques have been used to coat some polyurethane varnishes to form hard, brittle, inelastic coatings, they have not been used to form soft, flexible self sustaining films. One reason they probably have not been used to form flexible polyurethane films is that such films are essentially non-crystalline and under conventional thinking would be prepared from non-crystalline prepolymers which would not be grindable to a powder.
Soft, flexible polyurethane films have been formed by solution coating. One of the problems that is encountered when forming soft polyurethane films from solution is that as the thickness of the film increases, solvent bubbles form and cause pin holes in the film as the solvent evaporates. This makes it difficult to use polyurethanes in applications where a pin hole free film is required, as in the case of manufacturing surgical gloves, prophylactics, catheter balloons, etc.
A pin hole free polyurethane film has been obtained in a two-coat process wherein first a very thin primer film of polyurethane prepolymer is coated from solution and cured, followed by powder coating to build up the film thickness. The primer film providing a base-upon which particles of polyurethane prepolymer can flow out into a continuous film. Thus, the process has the effect of forming two films which prevent pinholes from extending from one face of the film to another. The process, however, suffers the inefficiencies in providing two different and independent coating solutions, separate cures after the first and second coats, and two prepolymer supplies, one a solution and the other a powder. Hence, there is a need for a more efficient process for producing pin hole free polyurethane films, and, particularly, a process that can be performed at one coating station.